If the complement proteins are always present in the body, then why are the results of the complement system not always occurring?

• A. The complement system proteins need to be activated by the invasion of a microbe.
• B. The complement system proteins are held in the lymph nodes until an infection occurs.
• C. The complement system proteins are inactive until cleaved by another complement protein.
• D. The complement system proteins are not always present and only translated when an infection occurs.
• E. A and B.
• F. A and C.
• G. D only.

Answer: (F)

Verified step by step guidance

1. Understand the role of the complement system: The complement system is a part of the immune system that enhances the ability to clear microbes and damaged cells, promote inflammation, and attack the pathogen's cell membrane.
2. Recognize the activation requirement: Complement proteins are present in the blood in an inactive form and require activation to function. This activation typically occurs through a cascade triggered by the presence of pathogens.
3. Identify the activation mechanisms: The complement system can be activated via three pathways: the classical pathway, the lectin pathway, and the alternative pathway. Each pathway involves the cleavage of complement proteins to activate them.
4. Clarify the role of cleavage: In the context of the problem, option C is correct because complement proteins are inactive until they are cleaved by another complement protein, which is part of the activation process.
5. Evaluate the options: Option A is also correct because the complement proteins need to be activated by the invasion of a microbe, which aligns with the requirement for activation. Therefore, the correct answer is A and C.